JPH04173939A - Ferritic stainless steel excellent in high temperature strength and toughness - Google Patents

Abstract

PURPOSE:To improve the high temp. strength and toughness of a stainless steel by preparing a ferritic stainless steel contg. specified ratios of C, Si, Mn, Ni, Cr, Al, Ti, Nb, V, Mo, N and Mg and in which the content of O as impurities is prescribed. CONSTITUTION:A ferritic stainless steel contg., by weight, <=0.02% C, 1.1 to 2.5% Si, <=1% Mn, <=1% Ni, 20 to 30% Cr, 0.5 to 3% Al, 0.01 to 0.4% Ti, 0.01 to 0.7% Nb, 0.01 to 0.5% V, 0.01 to 3% Mo, 0.001 to 0.05% N, 0.0001 to 0.05% Mg and the balance Fe with inevitable impurities and in which the content of O is regulated to 0.005% is prepd. In this way, the stainless steel having high temp. strength and excellent cold toughness and ductility and furthermore excellent in oxidation resistance can be obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is applicable to various oxidation-resistant members that have high high-temperature strength, excellent room-temperature toughness and high-temperature ductility, and are used in fields such as boilers, nuclear power, and the chemical industry; This invention relates to ferritic stainless steel suitable as a material for corrosion-resistant and oxidation-resistant parts for automobile exhaust gas.

(Prior Art) Ferritic stainless steel has the following characteristics in -g compared to austenitic stainless steel.

1) High thermal conductivity and low coefficient of thermal expansion; 2) Resistant to peeling of oxide scale; 3) Excellent thermal fatigue properties. 4) Resistant to stress corrosion cracking.

Ferritic stainless steel with these characteristics is
It has been widely used as heat-resistant steel, corrosion-resistant steel, and oxidation-resistant steel in industrial fields such as boilers, nuclear power, and chemical industries, where it is processed into plates, pipes, etc. Among them, An and S
Cr-3i-AI containing i and having a Cr content of 20 to 30%! Ferritic stainless steels produce a protective film of ^1201 and SiO□, which is more stable than Cr oxide, on the steel surface, so they have much better heat resistance and oxidation resistance, and are approximately 1.
It is used as a material for members used in a high-temperature oxidizing atmosphere of 1,000°C, such as gas piping, heat exchanger pipes, and pipes for automobile exhaust gas purification equipment.

However, Cr-5i-AI! Ferritic stainless steels have poor room-temperature toughness and ductility, making them difficult to process into pipes and plates, and they also have problems when used as structural materials because they have poor processability and impact resistance. Also,
Temperatures above 1000°C cause a rapid decrease in strength,
Since its strength is much lower than that of austenitic stainless steel, there are major restrictions on the fields in which it can be used.

By the way, Cr-5i-Aj! Several methods are known to improve the room temperature toughness and high temperature strength of ferritic stainless steels. For example, regarding room temperature toughness,
After keeping C and N in the steel low, T1, Nb, ■
It is known that it is effective to add Regarding high-temperature strength, it is disclosed in JP-A-1-287253 that it is effective to add appropriate amounts of Nb and (2). However, even if such a method is adopted, the effect of improving room temperature toughness and high temperature strength is not so great, as shown in the examples described later, and it is difficult to use the method for large members such as heat-resistant steel pipes, which are the object of the present invention. The reason for this is that it does not have sufficient room temperature toughness and high temperature strength at xoooo''C.
This was revealed from the test results conducted by the present inventors. Such steels with poor room temperature toughness and high temperature strength, for example, may not be able to be made into pipes, or even if they can be made into pipes, problems will occur when they are used as large components such as heat-resistant steel pipes. Specifically, when a tube after hot working is further subjected to cold working such as cold drawing, if the tube does not have sufficient room temperature toughness, cracks will occur in the tube, making it impossible to manufacture the tube. If the high-temperature strength is poor, the member is likely to deform or explode during use at about 1000°C. Furthermore, if the high-temperature strength is poor, it becomes difficult to reduce the weight of the member. In recent years, efforts have been made to improve energy efficiency and make equipment and equipment lighter and more compact in fields such as boilers, nuclear power, and the chemical industry.
There is a demand for ferritic stainless steel with excellent high-temperature strength that can be made thinner, but with conventional ferritic stainless steel, when the wall thickness of a member is made thinner, the high-temperature strength decreases.

(Problem to be solved by the invention) This invention! IB aims to improve the room temperature toughness and high temperature strength without impairing the excellent properties of the Cr-3i-Al ferritic stainless steel containing A2 and Si and having a Cr content of 20 to 30%. 0 The specific goal of the present invention is to have better high-temperature oxidation resistance than existing ferritic stainless steels, and a high-temperature tensile strength of 2 kgf at 1000"C.
The object of the present invention is to provide a ferritic stainless steel having properties of 2 kgf-m/cm" or more, an elongation of 30% or more, and a normal temperature tensile strength value of 2 kgf-m/cm" or more.

(Means for Solving the Problems) The present inventors have developed a method that contains Al and Si and has a Cr content of 20 to 30%.
As a result of a detailed study of the influence of alloy components on the room temperature toughness and high temperature strength of ferritic stainless steels, the following findings were obtained.

(2) Mg is an effective component for improving room-temperature toughness, and when Mg is added after suppressing 0 (oxygen) in steel to 0.0050% or less, room-temperature toughness is significantly improved.

■ However, the effect of Mg can only be obtained in the presence of ■, Ti, Nb, and It is necessary to add Ti, Nb and Mo in combination.

(2) Further, the combined addition of Mg, V, Tr, Nb and Mo also improves high temperature strength without impairing ductility and thermal fatigue properties.

This invention was completed based on the above knowledge, and its gist is the following ferritic stainless steel.

(i) In weight%, C: 0.02% or less, Si: 1
．． 1 to 2.5%, Mrz 1% or less, Ni: 1% or less, C
r: 20-30%, Aj! : 0.5-3%, Ti:
0.01-0.4%, Nb: 0.01-0.7%,
V: 0.01-0.5%, Mo20, 01-3%,
N: 0.001-0.05%, Mg: 0.0001
A ferritic stainless steel with excellent high-temperature strength and toughness, containing up to 0.05%, the remainder consisting of Fe and unavoidable impurities, and containing 0.005% or less of O (oxygen) as an impurity.

(ii) In addition to the component (i) above, further 0.000
A ferritic stainless steel containing 1 to 0.02% by weight of B and having excellent high-temperature strength and toughness.

(ij) A ferritic stainless steel having excellent high-temperature strength and toughness, which further contains 0.01 to 1% of Cu in addition to the components (i) or (ii) above.

(iv) In addition to the above components (i), (11) or (mountain), 0.01 to 0.2% by weight of La5
A ferritic stainless steel containing one or more selected from Ce, Zr, Y, Ta, and Ca and having excellent high-temperature strength and toughness.

(Function) Hereinafter, the function and effect of each alloy component of the steel of the present invention and the reasons for limiting their content will be explained.

C: C is an austenite stabilizing component, and it combines with Cr to form carbides and reduces the amount of solid solution Cr, which is effective for oxidation resistance, so it is desirable to keep its content as low as possible. If the content exceeds 0.02%, not only will carbide precipitation increase, but also toughness, weldability, and workability will be significantly impaired, especially if the steel becomes martensite during cooling after austenite transformation at high temperatures. , the upper limit was 0.02%.

Si: Si forms a stable protective film of SiO□ on the steel surface,
It has the effect of increasing oxidation resistance. In addition, Si is A/! in AN-added steel! ,0. It also has the effect of preventing the protective film from peeling off. If the content is less than 1.1%, the above effects cannot be obtained, and if it exceeds 2.5%, toughness, weldability, and workability are impaired, so the content was set at 1.1 to 2.5%. .

Mn: Mn is added in an appropriate amount for deoxidizing and desulfurizing the steel, but since excessive addition impairs oxidation resistance, the upper limit of its content is set at 1%.

Ni: Ni is an austenite stabilizing component, and if it is in a small amount, it contributes to improving toughness. However, if the content exceeds 1%, residual austenite will be generated and the toughness and strength will be impaired, so the upper limit was set at 1%.

Cr: Cr is an essential component for ensuring oxidation resistance and high temperature corrosion resistance. However, if the content is less than 20%, 10
Sufficient oxidation resistance cannot be obtained for use at temperatures around 00℃,
On the other hand, even if it exceeds 30%, no further improvement effect appears,
On the contrary, since it causes a decrease in workability and toughness, its content was set at 20 to 30%.

A2: Like Cr and St, ^l is an essential component for ensuring oxidation resistance. A1 forms a protective film of Al, O, on the steel surface to improve high temperature oxidation resistance. Its effect is particularly noticeable at high temperatures of 900"C or higher, and the higher the temperature, the better the oxidation resistance compared to steel with only Cr added. However, if the content is less than 0.5%, the above effect is lost. If it exceeds 3%, workability and toughness are impaired, so the content was set at 0.5 to 3%.

Ti: Ti combines with C and N to form Ti(C,N)(7) precipitates to increase toughness and strength. This is Ti(
This is because the precipitation of C, N) stabilizes C and N in the matrix and refines the crystal grains. However, if the content is less than 0.01%, the above effect cannot be obtained, and if it exceeds 0.4%, inclusions will increase and the toughness will decrease significantly. .4%.

Nb: Like Ti, Nb combines with C and N to form Nb(C,
N) precipitates are formed to refine crystal grains and reduce solid solution C and N in the matrix to increase toughness and strength. However, if the content is less than 0.01%, the above effects cannot be obtained, and if the content exceeds 0.7%, a large amount of intermetallic compounds will precipitate, impairing toughness.
The content of

V; ■ is an important component from the viewpoint of ensuring high temperature strength, and T
Compared to i and Nb, it is difficult to precipitate as V(C,N), but in steels with trace amounts of Ti and Nb added and a slightly larger amount of N, fine precipitates of VN are formed and the high-temperature strength is improved. was found to significantly improve. In particular, this VN is finely precipitated by heat treatment at 600 to 800°C, so it cannot be used with Ti and Nb-added steels in which N is almost fixed as Ti(C,N) and Nb(C,N) during processing. is different. The effect of the above (■) is small if it is less than 0.01%, but on the other hand, if it exceeds 0.5%, the effect is saturated and the toughness is actually impaired, so the content was set at 0.01 to 0.5%. MO: Mo is a solid solution strengthening component, and is particularly effective for high-temperature strength at 1000"C. Mo not only increases the strength of steel without impairing its ductility and workability, but also improves its thermal fatigue properties. It is an extremely effective ingredient. However, its content is 0.0
If the content is less than 1%, the above effects cannot be obtained sufficiently, and if it exceeds 3%, a large amount of intermetallic compounds will precipitate, impairing toughness, workability and weldability. did.

N: Reducing solid solution N improves toughness. Also, Nb(C,
N), excess N above that stabilized as Ti(C,N)
forms VN and improves high temperature strength.

In the present invention, an excess of N is added in consideration of N which is normally stabilized by Ti and Nb. N content is 0.0
If it is less than 0.01%, fine precipitation strengthening of VN cannot be obtained;
If it exceeds 5%, the toughness will be significantly impaired, so 0.00
The content was set at 1 to 0.05%. The desired amount is 0
．． 02-0.03%.

Mg: Mg is a particularly important component in the present invention. It has long been known that Mg has the effect of cleaning steel by combining with O and S, but in addition to this effect, M increases the room temperature toughness,
It also has the effect of improving high-temperature strength, ductility, and thermal fatigue properties, and these effects can only be obtained when it is added in combination with V, Ti, Nb, and Mo, and the amount of oxygen is adjusted to 0.005% or less. I understand. However, if the Mo content is less than 0.0001%, the above effect cannot be obtained;
If the content exceeds 5%, workability and toughness will be impaired, so the content was set at 0.0001 to 0.05%.

0 (oxygen): Since 0 impairs the effect of Mg, the lower the content, the better.If more than 0.005% of 0 is contained,
The added Mg is consumed as MgO, and not only does the effect of Mg become impossible to obtain, but also inclusions increase and the strength, toughness,
Since it impairs weldability, the upper limit was set at 0.005%.

In addition to the above-mentioned components, the following components can be added as necessary.

B: When added in a small amount, B finely disperses carbonitrides, strengthens grain boundaries, and increases high-temperature strength. However, if the content is less than 0.0001%, the above effect is small, and 0.
．． If B exceeds 0.02%, toughness and workability will be impaired, so when adding B, the content should be 0.0001 to 0.02%.
It is preferable to set it in the range of 0.02%.

Cu: Cu has the effect of increasing high temperature strength and corrosion resistance.

A content of less than 0.01% does not have this effect, and a content of more than 1% impairs hot workability and high-temperature ductility. It is better to have a range.

La, Ces, Zr, Y, Ta and Ca: These components combine with C, S, N, 0 to improve the toughness of steel. However, if the content is less than 0.01% of each, the above-mentioned effects will not be obtained, and if the content exceeds 0.2% of each, inclusions will increase and the toughness and workability will be impaired, so these components are added. In this case, the content of each is preferably 0.01 to 1%.

In addition to the above-mentioned components, the steel of the present invention contains Fe and unavoidable impurities as the remainder. Typical impurities are P and S, both of which are components harmful to toughness and high-temperature ductility, so P should be in the range of 0.025% or less and S in the range of 0.01% or less. It is best to minimize it as much as possible.

(Example) Steel having the chemical composition shown in Table 1 was melted in a 150 kg vacuum melting furnace, and the ingot was forged at 50 to 950°C to form a plate with a thickness of 25 mm. These plates were further heated to 1150°C.
After heating, it was hot rolled into a plate with a thickness of 7 m.

The finishing temperature of hot rolling was 1000''C, and the plate after rolling was air cooled to room temperature.

A steel is 5US430 ferritic stainless steel specified by JIS, B and C are 18 (:r series and Si
and Comparative steels with addition of Al2, Classes D to F are 24Cr
-1,55i-1,5Al comparative steels, G steel to Z class are steels of the present invention.

A round bar tensile test piece (φ6
+u+
5mm) and a plate-shaped oxidation resistance test piece (t2X 2
1125 mm) was cut out and tested.

The tensile test was conducted at 1000°C using the JIS high temperature tensile test method, and the tensile strength at that temperature was 0.2.
% proof stress and elongation were measured. Charpy impact test
The impact value at room temperature (20°C) was measured. The oxidation resistance test was conducted by repeatedly heating the specimen to 1000°C in the atmosphere for 1 hour and cooling it in the air 500 times. After the test, the specimen was descaled and the weight loss was measured. Oxidation resistance was evaluated.

Table 2 summarizes the results of these tests.

(Hereinafter, blank spaces) Table 2 (Test Results) From Table 2, it can be seen that the steels of the present invention from G steel to zw4 are superior in all properties to the comparative steels from A steel to Fl. In other words, the tensile strength at 1000°C of the comparative steel is 2 kgf/
sm2, whereas the steel of the present invention has a
The tensile elongation at break at 1000°C is less than 30% for the comparison steel, while the tensile elongation at break for the invention steel is 30%.
That's all. Room temperature toughness of all comparison steels is 11tgf-
m7cm", whereas the steels of the present invention have a thickness of 2 kgf-m7cm" or more. Regarding oxidation resistance, all of the steels of the present invention have an oxidation loss of 50 mg/cm" or less, which is better than SUS 430 steel of steel type A, and the 2dCr type steel has the best oxidation resistance among existing steels. D of
- It is equivalent to or even better than steel to F steel.

(Effects of the Invention) As shown in the examples, the ferritic stainless steel of the present invention has high high temperature strength, excellent room temperature toughness and ductility, and is also superior in oxidation resistance to conventional steel. This steel is high-temperature resistant and used in boilers, nuclear power, chemical industries, etc.
It can be widely applied to pipes, plates, and forged products as materials for oxidation-resistant members. It can also be applied to oxidation-resistant materials for automobile exhaust gas, which require repeated oxidation resistance and thermal shock resistance.

Claims (4)

[Claims] (1) In weight%, C: 0.02% or less, Si: 1.1~
2.5%, Mn: 1% or less, Ni: 1% or less, Cr: 2
0-30%, Al: 0.5-3%, Ti: 0.01-0
．． 4%, Nb: 0.01-0.7%, V: 0.01-0
．． 5%, Mo: 0.01-3%, N: 0.001-0.
05%, Mg: 0.0001 to 0.05%, the remainder consists of Fe and inevitable impurities, and O as an impurity.
Ferritic stainless steel with an oxygen content of 0.005% or less and excellent high-temperature strength and toughness. (2) In addition to the components of claim (1), further 0.0001
A ferritic stainless steel containing ~0.02% by weight of B and having excellent high-temperature strength and toughness. (3) In addition to the components of claim (1) or (2),
A ferritic stainless steel containing 0.01-1% Cu and having excellent high-temperature strength and toughness. (4) In addition to the components of claim (1), (2) or (3), each of 0.01 to 0.2% by weight of La and Ce
A ferritic stainless steel containing one or more selected from , Zr, Y, Ta, and Ca and having excellent high-temperature strength and toughness.